1. Field of the Invention
The present invention relates to devices, such as inductance type proximity sensors, for detecting the presence of an object, and more particularly to techniques for avoiding one device from interfering with operation of a similar nearby device.
2. Description of the Related Art
Proximity sensors are commonly used along assembly lines to detect the presence of a workpiece passing nearby and then activate equipment that performs manufacturing operations on the detected workpiece.
One common type of sensor has a transducer coil connected to a tuning capacitor to form a resonant circuit of a free running oscillator. The transducer coil generates an electromagnetic field across the path along which the workpieces travel. As a result of the electromagnetic field, eddy currents form in a conductive workpiece approaching the transducer coil and have a magnitude that is proportional to the magnetic flux lines impinging the workpiece surface. The eddy currents alter the inductance of the transducer coil and the series resistance of the coil and its core. The change in the impedance affects the quality factor Q of the tuned circuit causing the peak-to-peak oscillator voltage to decrease in proportion to the quality factor change. Typically the loading of the transducer coil by a workpiece of ferrous metal causes circuit to stop oscillating. Thus the presence of a metallic object can be determined by monitoring whether the circuit is oscillating.
In many industrial applications, several proximity sensors are placed near to each other. This can result in the electromagnetic field of one sensor interfering with the operation of an adjacent sensor. Such interference is commonly referred to as “cross-talk.” A common solution for cross-talk is to use proximity sensors having resonant circuits tuned to oscillate at different frequencies. However, this requires the design and manufacture of numerous models of the same type of sensor with differently tuned circuits.
It is desirable to have a technique that mitigates the effects of crosstalk between proximity sensors without requiring differently tuned circuits.